In optical communications the technique of wavelength-division multiplexing (WDM) allows several different light signals to be transmitted through a single optical fiber using a different wavelength for each signal. Implementation of WDM requires multiplexing techniques that spatially superimpose the light components or component beams of different wavelengths prior to transmission to form a single light beam. At the receiving end, WDM requires demultiplexing techniques that spatially separate the different light components of the single light beam to recover the original component beams.
In general, demultiplexing requires apparatus and methods for dispersing or spatially separating light into light components based on their constituent or component wavelengths. Conventional prisms or diffraction gratings are one means to select and separate different wavelengths of light. These conventional devices, however, have small dispersion (change in propagation angle with respect to wavelength). For example, typical gratings and prisms exhibits angular dispersion figures of less than 1°/nm. Therefore, they have to be quite large in order to achieve sufficient spatial separation of light components at different component wavelengths. The prior art teaches to replace gratings and prisms by alternative elements for effective wavelength separation.
Smaller, integrated arrayed waveguide grating (AWG) routers have been developed. Since waveguides are very temperature sensitive, these integrated devices need to be temperature-stabilized during operation. They are especially useful for high-channel count multiplexers and demultiplexers justifying the expensive fabrication and operation.
For lower-channel count systems, multilayer thin-film stacks are more cost-effective, since they are much less sensitive to temperature drift and normally do not require temperature stabilization during operation. Such stacked structures are constructed of a number of layers, e.g., alternating layers of a higher and lower refractive index dielectric material and spacer layers, as necessary. The thicknesses of the layers and their refractive indices determine the optical properties of these structures.
There are many examples of multiplexers and demultiplexers in the prior art using the transmission and reflection properties of multilayer thin-film structures as a function of wavelength or polarization. Such transmissive or reflective thin-film structures are often referred to as thin-film filters, including, e.g., a thin-film edge filter built of alternating layers of dielectric and spacer layers as taught by Thelen in U.S. Pat. No. 4,373,782. In this filter the bandpass reflectance characteristic of transition wavelength edges is non-polarizing for radiation incident at a preselected non-normal angle. Thin film filters are also used in conjunction with broadband reflecting regions in wavelength selective optical switches, as described, e.g., by Scobey et al. in U.S. Pat. No. 6,320,996. Improvements to the performance of thin-film bandpass filters, e.g., reduction of ripple effects and improvements to bandpass transmission are further described by Cushing in U.S. Pat. No. 6,011,652 entitled “Multilayer thin film dielectric bandpass filter”. In U.S. Pat. No. 6,147,806 to Park et al. describes the use of dichroic mirrors for demultiplexing of light into three color components. In U.S. Pat. No. 6,396,632 Liu et al. teach a tunable optical filter and an optical modulator that use a conventional thin film optical filter whose thickness is adjusted with the aid of a piezoelectric layer. The use of thin film filters in conjunction with dispersive elements for multiplexing applications is taught by Boye et al. in U.S. Pat. No. 6,404,958. In U.S. Pat. No. 6,122,417 Jayaraman et al. teach the use of a stack of layers as a Fabry-Perot cavity to multiplex and demultiplex an optical laser signal containing several different wavelengths. In fact, Jayaraman employs a linear array of Fabry-Perot cavities as filters to construct a WDM multiplexer-demultiplexer. It should be noted that the reflection/transmission wavelengths of Fabry-Perot filters are determined based on the physical principles of resonant optical cavities by adjusting the longitudinal dimension of the cavity to control the radiation modes that are resonant in the cavity. All of these devices use the well-known transmission and reflection properties of thin-film structures. Since these properties only allow for the separation into two different light paths—a reflection and a transmission path—a different thin-film filter structure is needed for each wavelength component to be demultiplexed. Thus, for a higher channel count system, many different thin-film filters have to be cascaded resulting in numerous or complicated components and high cost.
To reduce the number of components in a thin-film demultiplexing system, prior art has also investigated the use of thin-film filters in special geometries. For example, U.S. Pat. No. 6,008,920 to Hendrix teaches multiple channel multiplexer/demultiplexer devices using a single constant, non-variable wavelength selective optical interference filter preferably made of tens of layers and forming several cavities. The apparatus uses the angle-shift property of the thin-film filter, wherein the wavelength-selectivity changes with changing angle of incidence. Hendrix uses a system, e.g., a solid glass wedge adjacent the filter, to vary the angle of incidence and thus achieve wavelength selectivity.
In U.S. Pat. No. 6,111,674 Bhagavatula teaches the use of an optical path length generator with a plurality of partially reflective surfaces to build a multiple reflection multiplexer and demultiplexer. The partially reflective surfaces reflect successive portions of the energy of each of the different wavelength signals along different length optical paths. These intermediate pathways are recombined by a lens to achieve demultiplexing.
In U.S. Pat. No. 6,404,947 Matsuda teaches the use of a photonic crystalline layer composed of a stack of layers made up of multiple fine lines for wavelength separation in a demultiplexer and demultiplexer-receiver. By changing the spacing of the fine lines along the layer, the change in the position of the band edge of this two-dimensional photonic crystal is used for demultiplexing. Although these structures are more compact than other prior devices, their fabrication is complex and expensive.
In order to increase the dispersion, prior art has also investigated the use of higher-dimensional structures, i.e., structures that have varying optical properties in two- or three-dimensions. Recently, Kosaka et al. (“Superprism phenomena in photonic crystals,” Phys. Rev. B, Vol. 58, No. 16, 15 Oct. 1998; “Self-collimating phenomena in photonic crystals,” Appl. Phys. Lett., Vol. 74, No. 9, 1 Mar. 1999; “Photonic crystals for micro-lightwave circuits using wavelength-dependent angular beam steering,” Appl. Phys. Lett., Vol. 74, No. 10, 8 Mar. 1999) have proposed a method based on photonic crystals that can give angular dispersion many times larger than a prism or diffraction grating by relying on the “anomalous dispersion effect” or the “superprism effect” observed for non-normal incidence light.
It should be noted that the dispersive effects of thin film stacks on one-dimensional or normal-incidence light have been studied. N. Matuschek et al., “Analytical Design of Double-Chirped Mirrors with Custom-Tailored Dispersion Characteristics”, IEEE Journal of Quantum Electronics, Vol. 35, No. 2 (1999), pp. 129-137 and M. Jablonsky et al., “The Realization of All-pass Filters for Third-order Dispersion Compensation in Ultrafast Optical Fiber Transmission Systems”, Journal of Lightwave Technology, Vol. 19, No. 8 (2001), pp. 1194-1205 discuss the theory and uses of temporal dispersion characteristics of thin film stacks acting as all-pass reflection filters. In order to improve the performance of their thin film stack mirrors Matuschek et al. teach the use of chirping a stack mirror, i.e., slowly increasing the volume ratio between the two different materials to reduce the reflection off the front of the stack by impedance matching.
B. E. Nelson et al., “Use of a dielectric stack as a one-dimensional photonic crystal for wavelength demultiplexing by beam shifting”, Optics Letters, Vol. 25, No. 20, Oct. 15, 2000, pp. 1502-1504 and U.S. application Ser. No. 09/778,327 to D. A. B. Miller et al. teach the use of a dielectric stack that relies on group velocity dispersion in accordance with the superprism effect to spatially separate light beams of different component wavelengths. In this case the group velocity dispersion occurring just outside the main reflection region of a multilayer stack of dielectrics is used for wavelength multiplexing and demultiplexing.
Although these teachings go a long way to improving the efficiency and spatial separation of wavelength components of light by using the superprism effect in multilayer dielectric stacks, further improvements in efficiency and spatial separation are desired. Hence, what is needed is a compact device using a multilayer stack for multiplexing and demultiplexing of light in accordance with the superprism effect. The device should exhibit improved spatial separation characteristics of the wavelength components as well as high efficiency. Furthermore, what is needed is a multiplexing and demultiplexing device that is both very compact and easily fabricated.